FIG. 1 is a schematic circuit diagram showing the structure of a prior art frequency converter. In the figure, reference numeral 1 denotes a power supply terminal to which a DC voltage Vcc is applied, reference numerals 2a and 2b denote transistors each used for input of a local oscillation (LO) signal, reference numeral 3a denotes a reference transistor, reference numeral 5 denotes a constant current source, reference numerals 6a and 6b denote load resistors, reference numeral 7a denotes a radio frequency (RF) signal input terminal, reference numerals 8a and 8b denote local oscillation (LO) signal input terminals, reference numeral 9 denotes a reference bias terminal, reference numerals 10a and 10b denote output terminals, reference numeral 11a denotes a transistor pair that consists of the LO signal input transistors 2a and 2b, and reference numeral 14 denotes a transistor for input of a radio frequency (RF) signal. As shown in FIG. 1, in the prior art frequency converter, an RF signal is applied to the emitter-grounded RF signal input transistor 14.
Next, a description will be made as to an operation of the prior art frequency converter.
A DC voltage Vcc, which is applied to the power supply terminal 1 of the prior art frequency converter as shown in FIG. 1, is then applied to the transistor pair 11a that consists of the two LO signal input transistors 2a and 2b and the RF signal input transistor 14, and the reference transistor 3a by way of the two load resistors 6a and 6b, respectively. The constant current source 5 supplies a constant current to a parallel circuit that consists of the transistor pair 11a and the reference transistor 3a by way of the RF signal input transistor 14. A bias current is supplied to the reference transistor 3a by way of the reference bias terminal 9.
The RF signal input transistor 14 amplifies an RF signal that is applied to the base terminal thereof by way of the RF signal input terminal 7a. On the other hand, the LO signal input transistor 2a amplifies an LO signal of positive phase that is applied to the base terminal thereof by way of the LO signal input terminal 8a, and the other LO signal input transistor 2b amplifies another LO signal of negative phase that is applied to the base terminal thereof by way of the other LO signal input terminal 8b. Thus, because the LO signal of positive phase and the other LO signal of negative phase are applied to the base terminals of the LO signal input transistors 2a and 2b, respectively, the transistor pair 11a in which the collector terminals of these transistors are commonly connected to each other and the emitter terminals of these transistors are commonly connected to each other generates a signal having a frequency that is two times as high as that of the LO signals.
The sum of an electric current that flows through the transistor pair 11a and an electric current that flows through the reference transistor 3a is equal to an electric current that flows through the RF signal input transistor 14, and is therefore equal to an electric current output from the constant current source 5 connected to the emitter terminal of the RF signal input transistor 14. Because while the signal generated by the transistor pair 11a and having a frequency that is two times as high as that of the LO signals changes the electric current that flows through the transistor pair 11a, the RF signal input transistor 14 operates from the constant current from the constant current source 5, a signal having a phase opposite to that of a change in the electric current that flows through the transistor pair 11a flows through the reference transistor 3a. Therefore, the prior art frequency converter as shown in FIG. 1 serves as an even order harmonics frequency converter having a gain, which mixes the RF signal amplified by the RF signal input transistor 14, the signal generated by the transistor pair 11a and having a frequency that is two times as high as that of the LO signals, and the negative-phase signal that flows through the reference transistor 3a, and outputs the mixed signal as a differential signal by way of the output terminals 10a and 10b respectively connected to the load resistors 10a and 10b. 
A problem with the prior art frequency converter constructed as previously mentioned is that the saturation and distortion characteristics of the frequency converter are determined by the large signal characteristic of the RF signal input transistor 14 that is an active element, and therefore the saturation and distortion characteristics cannot be easily improved and degrade remarkably when the prior art frequency converter operates from a low voltage and/or a low current.
The present invention is proposed to solve the above-mentioned problem, and it is therefore an object of the present invention to provide a frequency converter that can exhibit an excellent saturation characteristic and an excellent distortion characteristic even when operating from a low voltage and/or a low current because of a resistor that is a passive element and is disposed in an RF signal input circuit instead of a transistor.